This invention relates to a liquid/gas contacting apparatus.
Perforated trays with downcomers are one of the most commonly used internals in industrial columns or towers for contacting a liquid and a gas. In such columns, liquid flows across the trays and down through the downcomers from tray to tray, while gas permeates upwardly through the trays contacting and foaming liquid thereon.
Many of these columns, which may be used for distillation, absorption and stripping operations, involve having solids present in particulate form, see for example, xe2x80x9cSubdue Solids in Towerxe2x80x9d, A. W. Stoley et al, Distillation and Other Industrial Separations, pages 95-104, dated January 1995, published in Chemical Engineering Progress, Stoley et al, center column, lines 25-38, page 99, which states that xe2x80x9ca major problem with downcomers is the creation of dead spots near the tower wall, near outlet weirs, and along the tower shell at the bottom of the downcomerxe2x80x9d.
Various proposals have been made, see, for example, U.S. Pat. No. 2,646,977, dated Jul. 28, 1953, A. T. Lee et al, U.S. Pat. No. 4,749,528, dated Jun. 7, 1988, J. T. Lavin, and U.S. Pat. No. 6,003,847, dated Dec. 21, 1999, A. T. Lee et al, to reduce the tendency of liquid to dwell at the periphery of the tray adjacent the column wall so that distillation is equally effective at the peripheral and central areas.
While these proposals have achieved various degrees of success in avoiding sediment collecting, stagnant zones in the liquid adjacent the column wall, there remains the problem that friction between the liquid and the trays has a marked effect on reducing liquid flow adjacent the column walls. By reducing this friction a greater throughput of liquid through the apparatus would also be achieved.
There is a need for a liquid/gas contacting apparatus wherein the friction between the liquid and the tray is reduced so that the tendency of liquid to dwell adjacent the column wall is reduced, and greater throughput of liquid through the apparatus is also achieved.
Another problem that needs to be addressed, once the friction between the liquid and the tray has been reduced, is that while the flow of liquid across the entire tray has been increased, the fact that the columns, by convention, are upwardly extending cylinders, there is even more of a tendency, because of the reduced friction, for liquid to flow at a greater rate across a central portion of the tray, from one downcomer to the other, rather than along the segments of circle portions adjacent the column walls.
There is also a need for the liquid flow, across a tray of reduced friction, to be more evenly distributed across the width of the tray.
According to the present invention there is provided a liquid/gas contacting apparatus, comprising,
a) an upwardly extending casing,
b) a series of perforated tray assemblies partitioning the casing at different levels, and having drainage openings, for, in operation, causing liquid descending in the casing to flow across the tray assemblies and be frothed thereon by gas, to be contacted with the liquid, which is ascending in the casing and permeating the perforations, and wherein,
c) each tray assembly comprises,
i) an upper surface layer which is repellent to the liquid, and
ii) an underside, rough, surface layer which is wettable by the liquid.
Preferably, for each tray assembly, liquid deflecting ridges are provided in the upper surface layer, the ridges,
i) lying between perforations,
ii) having an upper surface layer which is repellent to the liquid, and
iii) extending along paths which slope outwardly, in substantially chevron patterns, in a downstream direction for liquid flow, from central portions of the tray assemblies.
Preferably the ridges have sloping sides.
Preferably, each tray assembly comprises a steel plate, a fluorocarbon polymer upper layer forming said upper surface layer and bonded to the plate, and an exposed, rough underside of the steel plate forming the underside, rough surface layer.
The fluorocarbon polymer may be selected from the group consisting of polytetrafluoroethylene, polymers of chlorotrifluoroethylene, fluorinated ethylene-propylene polymers, polyvinylidene fluoride and hexafluoropropylene.
The liquid deflecting ridges may each comprise a steel strip standing on one edge, on, and welded by welds on both sides to the steel plate of the perforated tray assembly, with the welds providing the said sloping sides.
The liquid deflecting ridges may each comprise an upwardly extending deformed portion of the steel plate of the perforated tray assembly.
The perforations may be lined with a surface layer which is repellent to the liquid.